Quartiles are used to provide relative ranking of values for river water quality indicators on LAWA to determine current state. Sites in Quartile 1 (Q1) are in the best 25% of comparable sites, whereas sites that rank in the worst 25% of comparable sites are grouped into Quartile 4 (Q4).
This is a relative measure, in that it determines the relative ranking of sites against other sites (i.e. sites are better or worse when compared to other sites). See factsheet on calculating water state for more